The present invention relates to an improvement in a process for the preparation of steroids. More particularly, the invention Is concerned with improvements in a process for preparing steroids, such as 3-oxo-4-azasteroids, by hydrogenation of the corresponding steroid alkene. Compounds of this type are known to be useful in the preparation of compounds having 5xcex1-reductase inhibitor activity.
Steroid 5xcex1-reductases catalyze conversion of testosterone to DHT in an NADPH dependent fashion as shown in Scheme A. 
The inhibition of the conversion of testosterone to DHT is anticipated to be useful in the treatment of a variety of androgen responsive diseases, e.g., benign prostate hyperplasia, prostate cancer, acne, male pattern baldness and hirsutism. Hence, 5xcex1-reductase inhibitors have been the subject of active research worldwide. For example, see: Hsla, S. and Voight, W., J. Invest. Derm., 62, 224 (1973); Robaire, B. et al., J. Steroid Biochem., 8, 307 (1977); Petrow, V. et al., Steroids, 38, 121 (1981); Liang, T. et al., J. Steroid Biochem., 19, 385 (1983); Holt, D. et al., J. Med. Chem., 33, 937 (1990); U.S. Pat. No. 4,377,584, U.S. Pat. No. 4,760,071 and U.S. Pat. No. 5,017,568. One particularly promising 5xcex1-reductase inhibitor is MK-906 (Merck), known by the generic name, finasteride, and marketed under the trademark, Proscar, is an inhibitor of type 2 5xcex1-reductase. In addition, dual inhibitors of type 1 and 2 human 5xcex1-reductase are disclosed in WO 95/07926 and WO 95/07927, the contents of which are incorporated herein by reference.
Processes for the preparation of the 5xcex1-reductase inhibitors are described in WO 95/07926, WO 95/07927, U.S. Pat. No. 4,760,071, U.S. Pat. No. 4,377,584, U.S. Pat. No. 4,179,453, U.S. Pat. No. 5,670,643 and Bhattacharya, A. et al., J. Am. Chem. Soc., 110, 3318 (1988). Important intermediates in the preparation of 5xcex1-reductase inhibitors are 4-aza-5xcex1-androstan-3-ones, e.g. 3-oxo-4-azaandrost-17xcex2-carboxylic acid, which can be prepared by hydrogenation of the corresponding 4-aza-androst-5-en-3-one, e.g. 3-oxo-4-azaandrost-5-en-17xcex2-carboxylic acid. WO 95/07926 and WO95/07927 describe a process by which a 17xcex2-substituted 4-aza-androst-5-en-3-one is converted to the corresponding 17xcex2-substituted 4-aza-5xcex1-androstan-3-one by hydrogenation. For example, the hydrogenation may be carried out in acetic acid at 60 to 70xc2x0 C. and 276-414 kPa (40-60 psi) hydrogen pressure in the presence of catalytic platinum oxide.
The problem to be solved by the present invention is the provision of a superior, and more selective process for the hydrogenation of steroid alkenes.
Scheme 1 shows the hydrogenation of a steroid, 3-oxo-4-azaandrost-5-en-17xcex2-carboxylic acid, to the corresponding 5xcex1-androstane and 5xcex2-androstane isomers. 5xcex1-androstane isomer produced using the reaction can be used in the preparation of 5xcex1-reductase inhibitors. For large scale manufacture, a hydrogenation process which could be carried out at atmospheric pressure was desired. However, as demonstrated by the experiments below, considerable problems relating to xcex1:xcex2 ratios were experienced when hydrogenation was carried out at atmospheric pressure. 
The present inventors have found a process for hydrogenating steroid alkenes which can be performed at atmospheric pressure. The problem of low xcex1:xcex2 ratios was solved by carrying out the hydrogenation in the presence of ammonium acetate, ammonium formate or ammonium propionate.
Accordingly, the present invention provides a process for hydrogenating steroid alkenes comprising the step of hydrogenating one or more double bonds in the presence of ammonium acetate, ammonium formate, ammonium propionate, or mixtures thereof and an appropriate catalyst.
A further aspect of the invention is the use of the process of the invention in the preparation of a 5xcex1-reductase inhibitor.